• A tolerance of about 25% of the adopt unit hydrograph duration is generally accepted, without serious errors.
  • Thus a 4 hours unit hydrograph might be applied for storms of 3 hours to 5 hours effective duration.
  • Large variation in the rainfall intensity during the unit period may considerably affect the accuracy of the unit hydrograph approach.
  • Errors due to this reason can be reduced by using unit hydrographs for relatively short time periods.
  • The short period of unit hydrographs can be used to develop the hydrograph resulting from a long rain of varying intensity.
  • Experience has shown that the best unit period is about one fourth of the basin lag i.e.
  • In general it has been found that the period of unit storm should always be less than the period of rise (probably half or so).
  • Sometimes the unit hydrographs having a similar range in durations are averag to obtain an average unit hydrograph.
  • An average unit hydrograph is preferred to a single storm unit hydrograph since the averaging tends to minimise the errordata.